Field of the Invention
The present invention relates, in particular, to a high strength copper alloy tube that is suitable to a heat exchanger using a cooling medium such as an HFC-based fluorocarbon or CO2 and is excellent in pressure fracture strength and processability.
Description of the Related Art
For example, a heat exchanger for air conditioners is primarily constituted with a U-shaped copper tube bent like a hairpin (hereinafter, copper tubes also include a copper alloy tube) and fins (hereinafter, referred to aluminum fins) made from aluminum or an aluminum alloy plate. More specifically, for the heat transfer part of a heat exchanger, a copper tube bent in a U-shape is passed through through-holes of aluminum fins and a jig is inserted into the U-shaped copper tube to expand the tube, thereby closely attaching the copper tube and the aluminum fin each other. Then, further, the open end of this U-shaped copper tube is tube-expanded and a bent copper tube similarly bent in a U-shape is inserted into the tube-expanded open end. The bent copper tube is brazed to the tube-expanded open end with a brazing material such as copper phosphor brazing filler metals, thereby being connected to make a heat exchanger.
Thus, a copper tube used for a heat exchanger requires thermal conductivity as basic properties as well as good bending workability and good brazing properties when the above heat exchanger is produced. Phosphorous-deoxidized copper which has appropriate strength has been widely used as a copper tube material which is good in the properties.
On the other hand, HCFC (hydro-chlorofluorocarbon)-based fluorocarbons have been widely used for cooling media used for heat exchangers such as air conditioners. However, HCFC has a large ozone depleting potential, so that HFC (hydrofluorocarbon)-based fluorocarbons with small values of the ozone depleting potential have come to be used recently from the viewpoint of earth environment protection. In addition, CO2 which is a natural cooling medium recently has come to be used for heat exchangers used for water heaters, automotive air-conditioning equipment, vending machines, and the like.
However, the condensing pressure during operation needs to be enlarged to use these HFC-based fluorocarbons and CO2 as new cooling media and maintain the same heat transfer performance as HCFC-based fluorocarbons. Usually, in a heat exchanger, pressures at which these cooling media are used (pressure of a fluid which flows in the heat exchanger tube of a heat exchanger) become maximum in a condenser (gas cooler in CO2). In this condenser or a gas cooler, for instance, R22 of HCFC-based fluorocarbons has a condensing pressure of about 1.8 MPa. On the other hand, in order to maintain the same heat transfer performance as R22, R410A of HFC-based fluorocarbons needs to have a condensing pressure of 3 MPa and the CO2 cooling medium needs to have a condensing pressure of about 7 to 10 MPa (supercritical state). Therefore, the operating pressures of these new cooling media increase by a factor of 1.6 to 6 as compared with the operating pressure of the conventional cooling medium R22.
However, heat exchanger tubes made from phosphorous-deoxidized copper have a small tensile strength, whereby the thickness of the heat exchanger tube needs to be large in order to strengthen the heat exchanger tube, corresponding to an increase in operating pressure of these new cooling media. Additionally, upon assembly of heat exchangers, the brazed part is heated to a temperature of 800° C. or more for a several seconds to tens of second, so that crystal particles are made bulky in the brazed part and its vicinity as compared with other parts, leading to a decrease in strength due to softening. As the results, when a heat exchanger tube made from phosphorous-deoxidized copper is used for a heat exchanger for a new cooling medium, the thickness needs to be larger than before. Accordingly, the use of phosphorous-deoxidized copper as a heat exchanger tube for a new cooling medium such as an HFC-based fluorocarbon or CO2 increases the mass of the heat exchanger by an amount of thickening of the heat exchanger tube, thereby raising the price.
For this reason, a heat exchanger tube which has a high tensile strength, excellent processability and good thermal conductivity is strongly demanded for thinning of the heat exchanger tube. In this respect, there is a definite relation between the tensile strength of a heat exchanger tube and its thickness. For example, when the operating pressure of a cooling medium which flows in a heat exchanger tube is set to be P, the outer diameter of the heat exchanger tube is set to be D, the tensile strength of the heat exchanger tube (in the longitudinal direction of the heat exchanger tube) is set to be σ and the thickness of the heat exchanger tube (bottom thickness in the case of the inner helically grooved tube) set to be t, the relation P=2×σ×t/(D−0.8×t) is present between them. When this equation is arranged as for t, t=(D×P)/(2×σ+0.8×P), showing that the larger the tensile strength of the heat exchanger tube, the smaller the thickness. When the heat exchanger tube is actually selected, a heat exchanger tube of the tensile strength and the thickness calculated by further multiplying the operating pressure P of the above cooling medium with the safety ratio S (normally, from about 2.5 to 4) is used.
A variety of copper alloy tubes such as Co—P-based and Sn—P-based copper alloy tubes which have strength higher than that of phosphorous-deoxidized copper have conventionally been proposed instead of phosphorous-deoxidized copper to satisfy the demand of the thinning of such a heat exchanger tube. For example, as the Co—P-based copper alloy tube, a seamless copper alloy tube for a heat exchanger which contains Co: 0.02 to 0.2%, P: 0.01 to 0.05% and C: 1 to 20 ppm, restricts a impurity of oxygen, has an excellent loading endurance of 0.2% and has an excellent fatigue strength has been proposed (see Japanese Patent Laid-Open No. 2000-199023).
In addition, as the Sn—P-based copper alloy tube, a copper alloy tube for a heat exchanger which contains Sn: 0.1 to 1.0% and P: 0.005 to 0.1%, restricts impurities such as O and H, is made from a composition to which Zn is selectively added and further has an average crystal grain size of 30 μm or less has been proposed (see Japanese Patent No. 3794971 and Japanese Patent Laid-Open Nos. 2004-292917 and 2006-274313).
On the other hand, as a technology to improve the fracture strength of heat exchanger tubes, a copper alloy tube for a heat exchanger to which alloy elements such as Al and Si are added has been proposed (see Japanese Patent Laid-Open Nos. 63-50439 and 2003-301250). Additionally, in a phosphor bronze copper alloy plate which has a large amount of Sn and is not an Sn—P-based copper alloy tube, it is well-known to specify a texture specified by X-ray diffraction intensity for improving the fracture strength of the plate (see Japanese Patent Laid-Open No. 2004-27331).